In recent years, with a growing usage of mobile apparatuses, an increasing number of mobile apparatuses became equipped with a touch panel with which a user can input certain signals by pressing parts of the touch panel corresponding to a specific signs on a display screen by a pen or a finger. Particularly, there are growing demands for lightweight plastic touch panels that are resistant to a dropping impact.
A conventional plastic touch panel is described below with reference to an accompanying figure using a transparent touch panel as an example. In the figure, the thickness of members is magnified to make the configuration more comprehensible.
FIG. 9 shows a cross-sectional view of a conventional touch panel. A transparent resin substrate 1 comprising a polycarbonate resin sheet or an acrylic resin sheet, and a first transparent film 2 comprising bi-axially oriented polyethylene terephthalate, are adhered to each other via a first transparent pressure sensitive adhesive layer 3.
On a top surface of the first transparent film 2, which is the surface opposite to the one that is adhered to the transparent resin substrate 1, a first transparent conductive layer 4 of indium tin oxide (hereinafter, referred to as ITO) is formed by sputtering. On the first transparent conductive layer 4, small-sized dot spacers 5 made of epoxy resin are disposed at a predetermined pitch.
A second transparent conductive layer 8 of ITO is formed beneath the bottom surface of a second transparent film 7. And the second transparent film 7 is facing to the first transparent film 2 in such a manner that the second transparent conductive layer 8 faces the first transparent conductive layer 4 with a predetermined space kept between the two layers. The first transparent film 1 and the second transparent film 7 are bonded to each other at an outer region 6 where insulating patterns, electrode patterns, wiring patterns and pressure sensitive adhesive material patterns are formed.
An inside of the outer region 6 constitutes a visible region 11 of the touch panel, where a user visibly identifies display patterns for operation of a display device and an apparatus, and operates these apparatuses.
The top surface of the second transparent film 7 is protected by a hard coat layer 9 for preventing from scratch.
A flexible circuit board 10 is adhered and fixed on the outer region 6, with another end of the circuit board 10 connected to an external circuit (not shown), to transmit signals led through from the first transparent conductive layer 4 and the second transparent conductive layer 8 to the external circuit.
An oriented polyethylene telephtalate (hereinafter, referred to as PET) film is preferably used for the films 2 and 7. Despite its low prices, PET can be easily provided with good properties including high heat-resistance and good adhesion to the transparent conductive layers especially when drawn to two directions (bi-axially oriented).
The following is a description of the operation of the conventional touch panel configured as described above.
First, when a given part of the top surface of the film 7 is depressed with a finger or a pen, the film 7 is partially bent downward with the part to be a center, and the corresponding part of the transparent conductive layer 4 comes in contact with the transparent conductive layer 8.
At this time, the rest of the layers which are not subject to the pressing operation remain in non-contact with each other due to the restriction of the dot spacers 5.
The position of the aforementioned part being pressed is detected by an external circuit through the flexible circuit board 10 by calculating a voltage ratio against predetermined voltages applied respectively on the transparent conductive layer 4 and the transparent conductive layer 8 at the contacting point.
However, the aforementioned conventional touch panel features a marked difference in thermal expansion coefficient between the transparent film 2 and the resin substrate 1. This is because the former uses an oriented film such as bi-axially oriented PET in which ITO film can be easily formed, while the latter is produced by methods such as extrusion or cast molding and does not undergo any particular orientation treatment. Therefore, heat treatment during a production and temperature and humidity change after the production of the touch panel may cause thermal expansion of the transparent film 2 and the transparent resin substrate 1 to generate differences in dimensions between the two films, resulting in a warp in the touch panel.
A warped touch panel is difficult to mount on an electronic apparatus. For instance, a touch panel of a size suitable for 4-inch liquid crystal display apparatus (approximately 90 mm×70 mm), may warp 0.5 mm or more after being kept in a high-temperature and high-humidity environment (60° C., 95% RH) for 500 hours. Due to this, a user in charge of mounting needs to mount it on an electronic apparatus firmly, and this in turn, makes it difficult to reduce mounting processes. Moreover, since levels of warp increase in proportion to a size of the touch panel, it has been difficult to produce a high quality touch panel which can be applicable to a liquid crystal display device (LCD) larger than 6 inches.